Nonvolatile semiconductor memory

ABSTRACT

Disclosed is a semiconductor memory having an internal booster, such as a flash memory, in which a situation that the program cannot escape from a writing operation can be avoided, and the writing operation can be promptly finished according to the level of an external source voltage. This semiconductor memory having an internal booster has a voltage detecting circuit (limiter LM) for detecting whether a boosted voltage has reached a predetermined potential or not and a timer capable of counting predetermined time. A control circuit applies the boosted voltage to a selected memory cell when the voltage detecting circuit detects that the boosted voltage has reached the predetermined potential and, when it is detected on the basis of counting information of the timer that the predetermined time has elapsed since the booster started the boosting operation, the control circuit applies the boosted voltage to the selected memory cell even if the boosted voltage generated by the booster has not reached the predetermined potential yet.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a technique especially effectivefor application to a nonvolatile memory having an internal booster, fromwhich stored information can be electrically erased. For example, theinvention relates to a technique effective for use in a flash memoryemployed for a portable electronic device.

[0002] In a portable electronic device, a volatile memory such as a DRAMand a nonvolatile memory such as a flash memory are used as memorydevices of data. On the other hand, in a portable electronic device, abattery is used as a power source, and there is always a danger ofoccurrence of a dead battery. Due to this, in the portable electronicdevices, a nonvolatile memory of which data is not erased even if thebattery runs down is becoming the mainstream.

[0003] A flash memory uses, as a memory cell, a nonvolatile memoryelement comprising a MOSFET of a double gate structure having a controlgate and a floating gate. By changing a fixed charged amount of thefloating gate, a threshold voltage of the MOSFET is changed, therebyenabling information to be stored. Such a flash memory needs a highvoltage (such as ±10V or higher) for changing the threshold voltage bypulling/injecting charges from/to the floating gate of the nonvolatilememory element in an operation of writing/erasing data to/from a memorycell. In the flash memory, the high voltage is generally generated by aninternal booster provided in a memory chip.

SUMMARY OF THE INVENTION

[0004] In an operation of writing/erasing data to/from a memory cell ina flash memory having an internal booster, the internal booster isactivated to boost an internal source voltage simultaneously with thestart of the writing/erasing operation. After the internal sourcevoltage is boosted to a predetermined voltage, a write voltage or anerase voltage is actually applied to a memory cell. Methods ofdetermining a timing of starting an actual writing/erasing operationafter the internal source voltage is boosted to a predetermined voltageinclude a method of determining the timing by monitoring a generatedvoltage and a method of determining the timing after elapse of apredetermined time on assumption that the internal source voltage hasincreased to the predetermined potential. In the former method, when theinternal source voltage does not reach the predetermined potential forsome reason, there is a fear that the program cannot escape from thewriting or erasing operation. Conventionally, the latter method ofdetermining the timing on the basis of time is generally employed.

[0005] In the method of determining the timing of actually applying thewrite voltage or erase voltage to the memory cell on the basis of time,however, the time required to boost the internal source voltage dependson the magnitude of an external source voltage, capability of thebooster, and the load capacity of word line, bit line, or the like. Thecapability of the booster and the load capacity can be calculated from adesigned circuit, and the external power source is determined in thespecification. The timing is determined by calculating an expected timein which the internal source voltage is sufficiently boosted also in thecase of performing the writing/erasing operation on a worst capacitor inthe circuit in a state where a source voltage of the lower limit valuedetermined in the specification (generally, a voltage lower than asource voltage used by about 10%) is applied.

[0006] In recent years, however, the variety of the external powersource of the memory is increasing. For example, external power sourcesoperating at 3.3V, 2.5V, 1.8V and the like are in demand. In a systemusing a memory, which is constructed on a substrate of a stay-at-homeapparatus such as a personal computer, the source voltage is fixed.Consequently, there is no problem to determine the timing by calculatinga time in which the internal source voltage is expected to be boosted bya booster with the source voltage (Vcc-10%) of the lower limit valuedetermined by the specification. On the other hand, the memory used inthe portable electronic device may operate on a DC voltage such as 3.3Vconverted from AC 100V, on a built-in battery of, for example, 1.8V, orthe like.

[0007] In this case, even if 1.8V is supplied, when the timing isdetermined by calculating the time in which the internal source voltageis expected to be sufficiently boosted also in the case of performingthe writing/erasing operation on the worst capacity in the circuit, thedata writing/erasing operation can be performed with accuracy without aproblem. In the case where the internal booster is constructed by usingan MOSFET, however, when the operation voltage is 1.8V, the draincurrent of the MOSFET is about {fraction (1/4)} of that when the voltageis 3.3V. Consequently, the time required to boost the internal sourcevoltage increases by four times. For example, the data writing operationis finished in 1 mS (millisecond) when the source voltage is 3.3V. Ittakes, however, 4 mS when the source voltage is 1.8V. In an actualproduct, the writing/erasing timing is determined on the basis of 1.8Vwith which the writing operation takes longer time even in the casewhere the external source voltage is 3.3V in consideration of thepossibility where the product is used with the external source voltageof 1.8V. Consequently, a problem such that the time required for thewriting/erasing operation when the external source voltage is 3.3V islong more than necessary occurs.

[0008] Further, the inventors of the present invention have found thatthe time required to boost the internal source voltage varies alsoaccording to a pattern of write data. Specifically, in association withan increase in packing density of a semiconductor memory, a bit linepitch of the memory array in a semiconductor memory is becoming veryhigh. A parasitic capacitance between neighboring bit lines is becominglarger than a capacitance parasitic on a bit line, which exists betweenthe bit line and the substrate, and a capacitance between the bit lineand a line extending above the bit line.

[0009] Moreover, since the flash memory generally adopts a method ofprecharging a bit line in accordance with write data and simultaneouslywriting all of memory cells connected to one word line (hereinbelow,referred to as one sector), there is a case that 210 bit lines areprecharged. In such a case, when all the write data of one sector is “0”or only one bit is “0” (since when all the write data is “1”, thewriting operation is not performed), precharging against the parasiticcapacitance between neighboring bit lines is not performed. When writedata is 1010101 . . . 10, every other bit line is precharged, that is,the charging against all the parasitic capacitance between bit lines isperformed. Consequently, the capacitive load on the booster is theheaviest in this case. By the deep examination on a flash memory beingdeveloped by the inventors of the present invention, it was found that avariation of about 1 mS at the maximum occurs in the boost time of theinternal power source in accordance with the kind of write data.

[0010] In the conventional method of determining the write start timingon the basis of time, since the timing has to be determined by using thecase of writing the data of “1010101 . . . 10” as the worst caseregarding the data, the write start timing has to be further delayed. Itwas clarified that, when the timing is determined in consideration ofthe worst case with the source voltage of 1.8V, a time allowance ofabout five times as long as the time necessary to boost the power sourcevoltage to write data of all “0” with the source voltage of 3.3V isnecessary, so that the write time takes very long.

[0011] The inventors then examined the method of monitoring not time buta boosted internal source voltage and, when the internal source voltagereaches a predetermined potential, starting the writing operation. Inthis case, however, when the internal source voltage does not reach thepredetermined potential by the cause such as a leak due to adhesion of aconductive foreign matter on a bit line, it is feared that the programcannot escape from the writing operation. It was also made clear thatthe method has a drawback that, by providing an internal source voltagemonitoring circuit constructed by a resistance dividing circuit and acomparator, an extra resistive load is placed on the booster, and thetime required to boost the internal source voltage increases.

[0012] An object of the invention is to provide a semiconductor memoryhaving an internal booster such as a flash memory, capable of avoiding asituation that the program cannot escape from the writing operation andpromptly finishing the writing operation in accordance with the level ofan external source voltage.

[0013] The above and other objects and novel features of the presentinvention will become apparent from the description of the specificationand the appended drawings.

[0014] The outline of a representative one of inventions disclosed inthe specification will be briefly described as follows.

[0015] There is provided a semiconductor memory having an internalbooster, comprising a voltage detecting circuit such as a limiter fordetecting whether a boosted voltage has reached a predeterminedpotential or not and a timer capable of counting predetermined time. Acontrol circuit applies the boosted voltage to a selected memory cellwhen the voltage detecting circuit detects that the boosted voltage hasreached the predetermined potential and, when it is detected on thebasis of counting information of the timer that the predetermined timehas elapsed since the booster started the boosting operation, thecontrol circuit applies the boosted voltage to the selected memory celleven if the boosted voltage generated by the booster has not reached thepredetermined potential yet.

[0016] More specifically, there is provided a semiconductor memoryhaving a plurality of memory cells and a booster for generating aboosted voltage on the basis of a source voltage supplied from theoutside, for applying the boosted voltage to a selected memory cell whenthe voltage generated by the booster reaches a predetermined level,comprising: a voltage detecting circuit for detecting whether theboosted voltage has reached a predetermined potential or not; a controlcircuit capable of controlling start and stop of a boosting operation ofthe booster; and a timer capable of counting predetermined time. Thecontrol circuit applies the boosted voltage to a selected memory cellwhen the voltage detecting circuit detects that the boosted voltage hasreached the predetermined potential and, when it is detected on thebasis of counting information of the timer that the predetermined timehas elapsed since the booster started the boosting operation, thecontrol circuit applies the boosted voltage to the selected memory celleven if the boosted voltage generated by the booster has not reached thepredetermined potential yet.

[0017] According to the means, also in the case where the level of anexternal source voltage is low and a voltage is not so boosted by thebooster, after elapse of predetermined time, data is written to a memorycell. Consequently, the situation that the program cannot be escapedfrom the writing operation since the boosted voltage does not reach apredetermined potential conventionally occurred can be avoided. When thelevel of the external source voltage is high and the boosted voltagereaches a predetermined potential in short time, the writing operationis started. Consequently, the writing operation can be promptlyfinished.

[0018] Desirably, switching means for supplying or interrupting theboosted voltage is provided. The switching means receives a controlsignal from the control circuit and applies the boosted voltage to theselected memory cell. With the configuration, when a plurality ofboosted voltages of levels different from each other such as the writevoltage and a verify voltage are necessary, without changing the levelitself of the voltage generated by the booster, the voltage to beapplied to a memory cell can be changed only by operating the switch.

[0019] Preferably, a clock generating circuit for generating an internalclock signal is provided. By using the circuit, it becomes unnecessaryto generate and supply a clock necessary for the operation of thebooster on the outside of the chip of the semiconductor memory, so thatthe burden on designing of the user is lessened.

[0020] The booster operates on the basis of a clock signal for boostinggenerated by the clock generating circuit, and the timer operates on thebasis of a clock signal for counting generated by the clock generatingcircuit. With the configuration, the clock generating circuit can becommonly used to generate the clock signal for boosting and the clocksignal for counting.

[0021] The semiconductor memory further has a plurality of word lines.Each of the plurality of memory cells is connected to a correspondingword line, and the boosted voltage is applied to a word line to whichthe selected memory cell is coupled.

[0022] Further, the booster has a positive voltage generating circuitfor generating a positive high voltage on the basis of a source voltagefrom the outside and a negative voltage generating circuit forgenerating a negative high voltage on the basis of the source voltagefrom the outside. With the configuration, not only the high voltagenecessary for writing but also the high voltage necessary for erasingcan be also generated on the inside of the chip.

[0023] Each of the plurality of memory cells has a threshold voltagecorresponding to data to be stored. Consequently, multi-valueinformation can be stored in a single memory cell.

[0024] Data is written/erased to/from each of the plurality of memorycells by using a tunnel phenomenon. In the writing/erasing method usingthe tunnel phenomenon, as compared with a method of injecting hotelectrons into a memory cell by passing a drain current, a highervoltage is generally needed, and a burden is caused by the booster.Consequently, by applying the present invention, the effect is enhanced.

[0025] Further, according to another aspect of the invention, there isprovided a semiconductor memory having a plurality of memory cells eachhaving a threshold voltage corresponding to data to be stored, aplurality of bit lines, a plurality of data latches provided for the bitlines, and a booster for generating a boosted voltage on the basis of asource voltage supplied from the outside, each of the plurality ofmemory cells being coupled to a corresponding bit line, the boostedvoltage being applied to the memory cell selected when the boostedvoltage reaches a predetermined potential, and data being rewritten tothe memory cell in accordance with data latched by the data latch,comprising: a voltage detecting circuit for detecting whether theboosted voltage has reached a predetermined potential or not; a controlcircuit capable of controlling start and stop of a boosting operation ofthe booster; and a timer capable of counting predetermined time, whereinthe control circuit applies the boosted voltage to a selected memorycell when the voltage detecting circuit detects that the boosted voltagehas reached the predetermined potential and, when it is detected on thebasis of counting information of the timer that the predetermined timehas elapsed since the booster started the boosting operation, thecontrol circuit applies the boosted voltage to the selected memory celleven if the boosted voltage generated by the booster has not reached thepredetermined potential yet.

[0026] According to the means, since the data latch is provided for eachbit line, the writing operation can be performed in a plurality oftimes. As described above, even when the writing operation is startedafter elapse of the predetermined time before the boosted voltagereaches the predetermined potential, by increasing the number of writingtimes, accurate writing can be guaranteed. Thus, the situation that theprogram cannot be escaped from the writing operation since the boostedvoltage does not reach the predetermined potential can be avoided. Inthe case where the level of the external source voltage is high, whenthe boosted voltage reaches a predetermined potential, the writingoperation is started. Consequently, the writing operation can bepromptly finished.

[0027] Desirably, each of the memory cells is constructed to storemulti-value information by having any one of a plurality of thresholdvoltages. With the configuration, without enlarging the area of thememory array, the storage capacity can be increased. In this case, ahigher boosted voltage is necessary as compared with the case of binarydata. By applying the invention, it can be prevented that the programcannot be escaped from the writing operation when the external sourcevoltage is low.

[0028] A second voltage boosted by the booster in accordance with datalatched by the data latch is applied to the bit line, and the controlcircuit reserves activation of the timer until the second voltagereaches the predetermined potential. When the writing operation isstarted before the potential of the bit line becomes sufficiently high,there is the possibility that a “disturb” failure that the thresholdvoltage of a not-selected memory cell changes occurs. Since the timer isstarted after the boosted voltage to be applied to the bit line hasreached the potential, even when the writing operation is started afterelapse of the predetermined time after that, the level of the boostedvoltage to be applied to the bit line is guaranteed, so that theoccurrence of the disturb failure can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a block diagram showing an example of a flash memory asone of semiconductor memories to which the invention is effectivelyapplied.

[0030]FIG. 2 is a circuit diagram schematically showing theconfiguration of a memory array.

[0031]FIGS. 3A and 3B are explanatory diagrams showing a distribution ofa threshold voltage in the case of storing binary data to a memory celland that in the case of storing four-value data.

[0032]FIG. 4 is a block diagram showing the primary constituents of asemiconductor memory according to the invention.

[0033]FIG. 5 is a circuit diagram showing the basic configuration of alimiter provided for a charge pump.

[0034]FIG. 6 is a flowchart showing an example of a write controlprocedure in the flash memory to which the invention is applied.

[0035]FIG. 7 is a block diagram showing the schematic configuration ofan internal power source circuit portion in the flash memory to whichthe invention is applied.

[0036]FIG. 8 is a circuit diagram showing an example of a capacitorparallel type charge pump for performing a boost of a first stage.

[0037]FIG. 9 is a circuit diagram showing an example of a capacitorserial type charge pump for generating a positive voltage, whichperforms a boost of a second stage.

[0038]FIG. 10 is a timing chart showing timings of operation clocks ofthe charge pump of FIG. 9.

[0039]FIG. 11 is a timing chart showing an example of a start timing ofa booster in a data writing operation on the flash memory to which theinvention is applied.

[0040]FIG. 12 is a block diagram showing the schematic configuration ofa boosted voltage supply circuit in FIG. 7.

[0041]FIG. 13 is a block diagram showing an example of the internalconfiguration of a storage medium such as a memory card, SmartMedia,memory stick, or the like in the case where the flash memory of theembodiment is used as a storage medium for a voice recorder.

[0042]FIGS. 14A to 14C are block diagrams each showing the schematicconfiguration of a voice recorder as an example of an applied systemusing a card-type storage medium.

[0043]FIG. 15 is a block diagram showing the schematic configuration ofan internal circuit of the voice recorder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Embodiments of the invention will be described hereinbelow withreference to the drawings.

[0045]FIG. 1 is a block diagram showing an example of a flash memory asan example of a semiconductor memory having therein a booster, to whichthe invention is effectively applied. Although not limited, a flashmemory FLM of the example takes the form of a multi-value memory capableof storing two-bit data to one memory cell, and is formed on a singlesemiconductor chip made of single crystal silicon or the like.

[0046] In the embodiment, a memory array is constructed by two mats.Between the two mats, a sense amplifier and latch circuit (hereinbelow,called a sense and latch circuit and indicated as SL in the drawing)which is connected to bit lines BL in each of the mats, and amplifiesand latches a read signal is disposed. On the outer sides of the mats,that is, the sides opposite to the sense and latch circuit SL over thebit lines BL, latches for temporarily storing read data are disposed.The latches will be called data latches hereinbelow, and indicated as DLin the drawing. The data latch on the upper mat side will be referred toas DLU and the data latch on the lower mat side will be referred to asDLD.

[0047] In FIG. 1, reference numeral 10 denotes a memory arrayconstructed by two memory mats MAT-U and MAT-D, and 20 indicates a dataconverting circuit for converting write data input from the outside inthe case of storing four-value data into four-value data (three bits)every two bits. In each of the memory mats MAT-U and MAT-D, memory cellseach constructed by a MOSFET of a double gate structure having afloating gate and a control gate are arranged in a matrix, control gatesof the memory cells in the same row are continuously formed to constructa word line WL, and drains of the memory cells in the same column can beconnected to a common bit line BL.

[0048] The memory array 10 has, in correspondence with the memory matsMAT-U and MAT-D, X address decoders (word decoders) 13 a and 13 b andword drivers 14 a and 14 b each for driving one of word lines WL in eachmemory mat to a selection level in accordance with corresponding one ofdecode results of the decoders 13 a and 13 b. Although not limited, inthe memory array 10 of the embodiment, the word drivers are provided onboth sides and in the center of each of the memory mats. A Y addressdecoder (Y-DEC) and column switches (CSW) which are selectively turnedon/off by the decoder (Y-DEC) and transfer data from the data convertingcircuit 20 to the sense and latch circuit are constructed integrallywith the sense latch column (SL) and the data latch column (DL).

[0049] In FIG. 1, the Y address decoder, column switch, and sense andlatch circuit are shown by a single function block 11 (Y-DEC & SL). Datalatch columns 12 a and 12 b for latching write data obtained byconversion of the data converting circuit 20 and read data from a memorycell are disposed on the outside (upper and lower sides in the drawing)of the memory mats. In the case of storing four-value data, three-bitwrite data obtained by the conversion of the data converting circuit 20is latched so as to be divided to the data latch columns 12 a and 12 band the sense and latch 11. In a reading operation, the data read fromthe memory cell is latched by the data latch columns 12 a and 12 b andthe sense and latch 11 and is properly subjected to logic computation,thereby being inversely converted to 2-bit data.

[0050] The flash memory of the embodiment has, although not limited, acommand decoder 31 for decoding a command supplied from an externalcontroller, and a control circuit (sequencer) 32 for sequentiallygenerating and outputting control signals to circuits in the memory toexecute a process corresponding to the command on the basis of thedecode result of the command decoder 31. When a command is given, thecommand decoder 31 decodes the command and automatically performs acorresponding process. The control circuit 32 has a ROM (Read OnlyMemory) in which, for example, a series of micro commands necessary toexecute a command is stored. The command decoder 31 generates a headaddress of the micro command group corresponding to the command andsupplies the head address to the control circuit 32 where the microcommands are sequentially executed to thereby generate control signalsfor circuits in the chip.

[0051] Other than the above circuits, the multi-value flash memory ofthe embodiment further has: a write/erase determining circuit 33 fordetermining whether a writing or erasing operation is finished or not onthe basis of data of the sense and latch column SL, notifying thecontrol circuit 32 of the result of determination, and finishing thewriting or erasing operation; a clock generating circuit 34 having anoscillator and a frequency divider, for generating a plurality of timingclocks necessary for operations of internal circuits, such as operationclocks CK1, CK2, CLK1, CLK2, and PCLK2 for a booster which will bedescribed hereinlater and a clock φc for a timer and supplying theclocks to the circuits in the memory; a status and test circuit 35having the functions of generating and outputting a ready/busy signalR/B in which the status of the memory is reflected and which indicateswhether the memory can be accessed from the outside or not and testingan internal circuit; a main amplifier circuit 36 for amplifying a signalread from the memory array 10; a power circuit 37; an input/outputcontrol circuit 38 for capturing an address signal, a write data signal,and a command input from the outside, supplying the captured signal to apredetermined circuit on the inside, and outputting a read data signalto the outside; an input/output buffer 39 for receiving a control signalinput from the outside and supplying the signal to the control circuit32 an other predetermined internal circuit; an address control circuit40; and a redundancy circuit 41 for replacing a defective bit in thememory array with a spare memory row.

[0052] The power circuit 37 includes a reference power voltagegenerating circuit for generating a voltage as a reference of a writevoltage or the like, an internal source voltage generating circuit forgenerating a voltage required on the inside of the chip such as a writevoltage, erase voltage, read voltage, or verify voltage on the basis ofa source voltage Vcc supplied from the outside, a power switchingcircuit for selecting a desired one of the voltages in accordance withthe memory operating state and supplying the selected voltage to thememory array 10, and a power control circuit 371 for controlling thosecircuits. Boosters 1 and 2 of the embodiment are provided in theinternal source voltage generating circuit in the power circuit 37. InFIG. 1, reference numeral 21 denotes a source voltage terminal to whichthe source voltage Vcc is applied from the outside, and 22 similarlyindicates a source voltage terminal (ground terminal) to which theground potential Vss is applied.

[0053] The address control circuit 40 includes an address counter ACNTfor receiving an address signal input from the outside and incrementingits count value, an address generator AGEN for automatically updating aY address at the time of data transfer and automatically generating an Xaddress at the time of data erasure, and a repair circuit for comparingan input address with a defective address and, when the addresses matchwith each other, switching a memory row or column to be selected.

[0054] Examples of control signals supplied from an external CPU or thelike to the flash memory of the embodiment are a reset signal RES, achip select signal CE, a write control signal WE, an output controlsignal OE, a command enable signal CDE indicative of a command, datainput, address input, or the like, and a system clock SC. The commandand address are supplied to the inside of the chip via the input/outputbuffer 39 in accordance with the command enable signal CDE and the writecontrol signal WE. Write data is supplied to the inside of the chipsynchronously with the system clock SC received in a state where thecommand enable signal CDE indicates the command or data input.

[0055]FIG. 2 shows a schematic configuration of the memory array 10. Aplurality of memory cells MC are arranged in a matrix in the memoryarray 10. The word line WL to which control gates of the memory cells inthe same row are connected and the bit line BL to which drains of thememory cells in the same column are connected are disposed so as tocross each other. The sources of the memory cells are connected to thecommon source line SL for applying the ground potential. The source lineSL is provided with a switch SW so that the sources of the memory cellscan be opened in a writing operation.

[0056] The sense and latch circuit 11 having the function of a senseamplifier for amplifying the potential of each of bit lines and a datalatching function is connected to one end of the bit line BL. A datalatch 12 a (12 b) having the data latching function is connected to theother end of each bit line BL. The data latch 12 a (12 b) is used tolatch data for changing the threshold voltage of a selected memory cellstep by step when the memory operates as a multi-value memory. Each ofthe sense and latch circuit 11 and the data latch 12 a (12 b) has aswitching device for electrical connection/disconnection to/from acorresponding bit line and means for discharging the bit line. The senseand latch circuit 11 may be provided with an inverting circuit forinverting the logic of data on the bit line. Since such switching deviceand inverting circuit are provided, in the case of four-value datastorage, data read from the memory cells while changing a read level aresubjected to a wired arithmetic on the bit line, thereby enabling thedata to be converted to two-bit data in the memory array.

[0057] Although not limited, the flash memory of the embodiment isconstructed so that binary data or four-value data can be stored in eachmemory cell. FIG. 3A shows a distribution of threshold voltages in thecase of storing binary data to each of memory cells and FIG. 3B shows adistribution of threshold voltages in the case of storing four-valuedata. In the case of storing binary data, verify voltages Vwvh and Vwvlare set so that the threshold voltage of a memory cell corresponding tostorage data “1” lies in a range of, for example, ±0.7V of 4.3V. Verifyvoltages Vevh and Vevl are set so that the threshold voltage of a memorycell corresponding to storage data “0” lies in a range of, for example,±0.7V of 1.5V. A read voltage Vr is set at an intermediate level of 2.9Vor the like.

[0058] In the case of storing four-value data, write verify voltagesVwvh4 and Vwvl4 are set so that the threshold voltage of a memory cellcorresponding to storage data “11” lies in a range of, for example,±0.4V of 4.8V. Write verify voltages Vwvh3, Vwvl3, Vwvh2, Vwvl3, Vwvh1,and Vwvl1 are set so that the threshold voltage of the memory cellcorresponding to storage data “10” lies in a range of, for example,±0.4V of 3.6V, the threshold voltage of the memory cell corresponding tostorage data “01” lies in a range of, for example, ±0.4V of 2.2V, andthe threshold voltage of the memory cell corresponding to storage data“00”, lies in a range of, for example, ±0.4V of 1.0V. Four-value readvoltages Vr1, Vr2, and Vr3 are set to levels such as 1.5V, 2.9V, and3.8V, respectively.

[0059] The gist of the invention will now be described by referring toFIG. 4. Shown in FIG. 4 are a central processing unit 321 (hereinbelow,called a CPU) of the control circuit 32, a ROM (Read Only Memory) 322 inwhich a control program and the like of the CPU 321 are stored, a chargepump 372 as a component of an internal source voltage generating circuitprovided in the power circuit 37, and a limiter 373 provided on theoutput side of the charge pump 372, for limiting a boosted voltage to adesired level. The limiter 373 has the function of outputting ahigh-level detection signal LMD when an output voltage Vup of thebooster reaches the desired level.

[0060] Since various limiters are already known, the limiter will not bedescribed in detail but its schematic configuration shown in FIG. 5 willbe briefly described. The limiter has a resistance dividing circuit 731constructed by two resistors R1 and R2 connected in series between theoutput terminal of the booster and the ground point, for generating avoltage obtained by dividing the boosted voltage Vup at a resistanceratio of the resistors R1 and R2, and a comparator 732 for comparing thevoltage generated by the resistance dividing circuit 731 with a presetreference voltage Vref of a predetermined level.

[0061] In the example of FIG. 4, an AND gate 375 for receiving, as inputsignals, an output of an AND gate 374 for controlling the clock CLK tothe charge pump 372 in response to a start signal ST from the CPU 321and the output signal LMD of the limiter 373 is provided, and the supplyof the clock signal CLK to the charge pump 372 is controlled by thelimiter output LMD. When the boosted voltage output from the charge pump372 becomes higher than the desired level, the operation of the chargepump 372 is stopped, so that excess power consumption can be reduced.

[0062] In the flash memory of the embodiment, the limiter output LMD issupplied to the CPU 321 in the control circuit 32 and, in response to achange to the high level of the limiter output LMD, the CPU 321 outputsa signal B to make the word driver 14 a start applying a write voltageto the word line WL.

[0063] Further, in the embodiment, the control circuit 32 is providedwith a timer 323. The CPU 321 sets set time read from the ROM 322 intothe timer 323 and starts the timer 323 simultaneously with a write startprocess. After elapse of a predetermined time, even if the detectionsignal LMD from the limiter 373 is not received, application of thewrite or erase voltage to the memory cell is started. The timer 323 hasa binary counter and performs a counting operation of counting a clockφc having a cycle of, for example, 50 nS (nanoseconds) supplied from theclock generating circuit (in the block 34 in FIG. 1) provided on theinside of the chip. In place of the timer having the binary counter forcounting the clock φc, a software timer for performing counting(incrementing the value of a register) by the operation of a program bythe CPU may be used.

[0064]FIG. 6 shows an example of a write control procedure of thecontrol circuit 32 in the flash memory to which the invention isapplied. In the embodiment, although not limited, the writing operationis performed by increasing the threshold voltage of a memory cell byapplying a high voltage to the word line WL (control gate) to injectnegative charges to the floating gate of the memory cell.

[0065] When a write command is received, the control circuit 32 assertsthe start signal ST to the charge pump 372 in the internal sourcevoltage generating circuit to start boosting (step S1). Subsequently,the control circuit 32 starts the timer 323 (step S2). Specifically, settime of the timer is read from the ROM 322 and set in the timer 323.After that, whether the detection signal LMD from the limiter 373 isreceived or not is checked in step S3. If NO, the program advances tostep S4 where whether the set time of the timer 323 has elapsed or notis determined. When the set time has not elapsed yet, the programreturns to step S3 and the determining operation is repeated.

[0066] When it is determined in step S3 that the detection signal LMD isreceived, or even if the detection signal LMD is not received, when itis determined in step S4 that the set time has elapsed, the programadvances to step S5 where the control signal B to start applying thewrite voltage is input to the word driver 14 a. In the following stepS6, a verifying operation of verifying whether the threshold voltage ofthe memory cell has changed to the predetermined level or not isexecuted. When the threshold voltage has changed to the verify voltage,a writing operation finishing process for setting “1” in a write end bitin a status register in the status and test circuit 35 is performed(step S7). By the process, the ready/busy signal R/B is changed to thelevel indicative of a ready status, so that the end of the writingoperation can be notified to external devices.

[0067] On the other hand, when the threshold voltage has not changed tothe verify voltage, the program moves to step S8 where whether thenumber of writing times has reached a preset number of times or not isdetermined. If NO, the program returns to step S1 and the process isre-started from the start of the internal power source. When the programadvances to step S8 again and whether or not the number of writing timeshas reached the preset number of times is determined, if it isdetermined that the number of writing times had reached the presetnumber of times before the writing operation is finished, the programmoves to step S9 where a write error process of setting “1” in a writeerror bit in the status register is performed.

[0068]FIG. 7 shows a schematic configuration of the internal powersource circuit of the flash memory having the booster according to theinvention. In FIG. 7, reference numeral 1 denotes a first booster forperforming a boost of a first stage on the basis of the source voltageVcc of 1.8V to 3.3V from the outside, and 2 indicates a second boosterfor performing a boost of a second stage on the basis of a voltage VWDPboosted by the first booster 1. In the embodiment, the first booster 1has two charge pumps CP12 and CP12 each of a capacitor parallel type.The second booster 2 has two charge pumps CP21 and CP22 each of acapacitor serial type.

[0069] Although not limited, the charge pumps CP11 and CP12 of thecapacity parallel type in the first booster 1 generate the boostedvoltage VWDP such as 7V and a boosted voltage VCP such as 5.5V,respectively, on the basis of the source voltage Vcc. Although thelimiter 373 and the control gate 373 are not shown in FIG. 7, each ofthe charge pumps CP11, CP12, CP21, and CP22 is provided with the limiterand the control gate corresponding to each boosted voltage.

[0070] In the capacitor serial type charge pumps CP21 and CP22 in thesecond booster 2, the charge pump CP21 generates a positive writevoltage VWW such as +18V to be applied to the word line WL as acapacitive load in the memory array 10 which is a nonvolatile memorydevice, and the charge pump CP22 generates a negative erase voltage VEWsuch as −16V to be applied to the word line WL. In the capacitorparallel type charge pumps CP11 and CP12 in the first booster 1, thevoltage VWDP boosted by the charge pump CP11 is supplied to the secondbooster 2 and the Y decoder & sense and latch circuit 11 for precharginga bit line BL in the memory array 10, and the voltage VCP boosted by thecharge pump CP12 is supplied to an internal logic gate circuit 4, a worddecoder 5, a voltage step-down circuit 6 constructed by a resistancedividing circuit or the like, for generating a read voltage to beapplied to the word line WL, and the like. The voltage step-down circuit6 generates a read voltage VR in a normal read mode, a verify voltageVVW in a write mode, and an erasure verify voltage VVE in an erasuremode.

[0071] The internal logic gate circuit 4 denotes, not a logic gatecircuit constructing an address buffer, a data buffer, or the likeoperated on the external source voltage Vcc, but a logic gate circuitwhich operates on an internal boosted voltage such as a decoder providedrelatively close to the memory array 10, a direct control circuit fordetermining an end of a writing/erasing operation, and a power switchingcircuit.

[0072] Further, the second booster 2 is provided with an amplitudedoubling circuit 3 for doubling the amplitude of the clock CK1. Theamplitude doubling circuit 3 is constructed to generate a clock PCLK1having an amplitude of 2VWDP on the basis of the boosted voltage VWDPgenerated by the booster 1.

[0073] The clocks CK1, CLK1, and CLK2 are supplied from the clockgenerating circuit (of the block 34 in FIG. 1) in the chip. The supplyof the clocks to the charge pumps CP11, PC12, CP21, and CP22 can beinterrupted by gates 7 a, 7 b, 7 c, and 7 d controlled by start signalsST1, ST2, ST3, and ST4 from the control circuit (block 32 in FIG. 1).When the supply of a clock is interrupted, the corresponding charge pumpstops the boosting operation.

[0074] In the embodiment of FIG. 7, reference numeral 8 denotes a worddriver (block 14 a in FIG. 1) for receiving a select signal from theword decoder 5 and driving a word line WL to a predetermined levelaccording to the operation mode, 9 a and 9 b denote voltage switches forselecting a voltage according to the operation mode and supplying theselected voltage to the word driver 8, and 9 c indicates a switch forselectively supplying the boosted voltage VWDP to the sense and latchcircuit 11 or the second booster 2 in the writing operation. The controlsignal B for starting the application of the write voltage supplied fromthe control circuit 32 to the word driver 14 a is used, for example, tocontrol the voltage switch 9 a.

[0075] As described above, in the embodiment, the booster 1 forperforming the boost of the first stage is constructed by the twocapacitor parallel type charge pumps CP11 and CP12. Consequently, thepower consumption can be reduced by stopping the operation of one of thecharge pumps in accordance with the operation mode. Even when the sourcevoltage is low, the control of generating the boosted voltage of adesired level with reliability can be performed.

[0076] Specifically, in the flash memory of the embodiment, the chargepump CP12 for generating the boosted voltage VCP supplies the operationpower to the internal logic gate circuit 4 and the voltage step-downcircuit 6. Consequently, the charge pump CP12 operates in any of theoperation modes, that is, all the time. Since a high boosted voltage isunnecessary in the read mode, the operation of the charge pump CP11 forgenerating the boosted voltage VWDP is stopped, thereby reducing thepower consumption. When the operation of the charge pump CP11 forgenerating the boosted voltage VWDP is stopped, the operations of thecharge pumps CP21 and CP22 in the booster 2 for performing the boost ofthe second stage on the basis of the boosted voltage VWDP are alsostopped. The operations of the charge pumps CP12, CP21, and CP22 can bestopped by interrupting the operation clocks supplied to the chargepumps CP12, CP21, and CP22 by the start signals ST2 and ST3.

[0077] In the embodiment of FIG. 1, the charge pumps CP21 and CP22 inthe booster 2 for performing the boost of the second stage generate thepositive boosted voltage VWW used for the writing operation and thenegative boosted voltage VWE used for the erasing operation,respectively. Therefore, by controlling to stop the operation of thecharge pump CP22 in the write mode and to stop the operation of thecharge pump CP21 in the erase mode, the power consumption can bereduced.

[0078] Further, in the embodiment, the charge pump CP11 for performingthe boost of the first stage on the basis of the source voltage isconstructed of the capacitor parallel type. Each of the charge pumpsCP21 and CP22 for performing the boost of the second stage on the basisof the boosted voltage VWDP generated by the charge pump CP11 isconstructed of the capacitor serial type. The charge pump of thecapacitor serial type has a characteristic that, although charges areconsumed by parasite capacity and a resultant voltage is rapidlysaturated when the number of stages increases, voltages applied tocapacitors for boosting at the stages disposed in series are almostequal to each other, and designing is easy from the viewpoint of awithstand voltage. On the other hand, the capacitor parallel type chargepump is not adapted to deal with a high boosted voltage from theviewpoint of a high withstand voltage since the voltages applied to thecapacitors arranged in parallel are various and become higher in stages.However, the capacitor parallel type charge pump has a characteristicthat even when the number of stages of capacitors is large to an extent,the boosted voltage is not easily saturated as compared with the chargepump of the capacitor serial type. Consequently, by using the chargepump of the capacitor parallel type as the charge pump for performingthe boost of the first stage and using the capacitor serial type as thecharge pump for performing the boost of the second stage as statedabove, as compared with the case where the types are opposite, arelatively high boosted voltage can be generated more efficiently.

[0079]FIG. 8 shows a schematic configuration of each of the charge pumpsCP11 and CP12 of the capacitor parallel type in the booster 1 forperforming the boost of the first stage.

[0080] The charge pump of FIG. 8 has capacitors C1 to C5 for boosting avoltage in five levels, arranged in parallel, switch MOSFETs Qs1 to Qs4provided between charge side terminals (nodes N1 to N5) of thecapacitors, a switch MOSFET Qs0 capable of applying the external sourcevoltage Vcc to the charge side terminal (node N1) of the capacitor C1 atthe first stage, a switch MOSFET Qs5 for transmitting the voltage VWDP(VCP) of the charge side terminal (node N5) of the capacitor C5 at thefinal stage to a circuit for receiving the voltage supply, andcapacitors Cb0 to Cb5 for instantaneously boosting a gate voltage toturn on the switch MOSFETs Qs0 to Qs5. The capacitance value of each ofthe capacitors Cb0 to Cb5 for instantaneous boost is set to besufficiently smaller than that of each of the capacitors C1 to C5 forboost. Each of the switch MOSFETs Qs0 to Qs5 functions as a diode bybeing turned on by the boosted voltage.

[0081] To reference side terminals (terminals opposite to the chargeside terminals) of the odd-numbered capacitors C1, C3, and C5 out of thecapacitors C1 to C5 for boost, the clock CLK1 having an amplitude of 0Vto Vcc is applied. To reference side terminals of the even-numberedcapacitors C2 and C4, a clock /CLK1 of the phase opposite to that of theclock CLK1, similarly having an amplitude of 0V to Vcc, is applied. Thereference side terminals of the odd-numbered capacitors C1, C3, and C5and those of the even-numbered capacitors C2 and C4 are alternatelyboosted by the clocks CLK1 and /CLK1 of opposite phases, and the switchMOSFETs Qs0 to Qs5 are controlled to operate like diodes, therebytransferring the charges in the capacitor C1 to the capacitors C2, C3,C4, and CS in this order to generate a boosted voltage. Although notlimited, the clock CLK1 has the cycle of 50 nS or the like.

[0082]FIG. 9 shows a schematic configuration of the charge pump CP21 forgenerating a positive boosted voltage in the capacitor serial typecharge pumps CP21 and CP22 in the second booster 2.

[0083] The charge pump of FIG. 9 has capacitors C11 to C13 forperforming a boost in three levels, connected in series via MOSFETs,switch MOSFETs Qs11 and Qs12 capable of connecting or interrupting thecapacitors, a switch MOSFET Qs13 for transmitting the voltage VWW on thecharge side terminal of the capacitor C13 at the final stage to acircuit for receiving the voltage supply, switch MOSFETs Qs21 to Qs23each connected between the charge side terminal (high potential sideterminal) of the capacitor and the source voltage terminal V0, switchMOSFETs Qs31 and Qs32 connected between the reference side terminals(low potential side terminals) of the capacitors C12 and C13 at thesecond and third stages and the ground potential Vss (0V), respectively,a switch MOSFET Qs24 connected between the gate terminal of the MOSFETQs13 at the final stage and the source voltage terminal V0, and abooster BST for boosting the gate voltage to sufficiently turn on theswitch MOSFET Qs13 at the final stage.

[0084] The booster BST is constructed by a MOSFET Qb and a capacitor Cbfor instantaneous boost connected in series between the charge sideterminal (high potential side terminal) of the capacitor C13 for boostand the gate terminal of the switch MOSFET Qs13 at the final stage, anda switch MOSFET Qs33 connected between a connection node Nb of Qb and Cband the ground potential Vss (0V). Qs11 and Qs12 out of the switchMOSFETs Qs11 to Qs13 and the MOSFET Qb in the booster are of thep-channel type. By applying the voltage V0 to their gate terminals, theyare turned off in normal times for the reason that the potentials of thesource and drain of the MOSFETs Qs11 and Qs12 are always the same or thepotential of the node on the ante stage side is higher than the other,and the potential at the node on the post stage side does not becomehigher. That is, by connecting the well areas of the MOSFETs Qs11, Qs12,and Qb to the nodes at the ante stage sides, a p-channel MOSFET havingan n-type well can be used. By forming the MOSFETs Qs11 and Qs12 of thep-channel type, the potential boosted at the ante stage can betransmitted to the post stage without a potential drop (drop in Vth).

[0085] On the other hand, the switch MOSFET Qs13 of the n-channel typeis used since the potential relation between the source and drain is notdetermined, and the booster BST is provided to transmit the voltageboosted at the ante stage to the post stage without a drop in potential(drop in Vth). Similarly, as the switch MOSFETs Qs21 to Qs24 forprecharging the capacitors by applying the voltage V0 to both terminalsof the capacitors C11 to C13 for boost, since the potential relationbetween the source and drain is not determined, MOSFETs of the N-channeltype are used. When the charge voltage is set as V0, the potential maydrop in an N-channel type MOSFET. Consequently, the clock PCLK1 havingan amplitude of 2V0 as shown in FIG. 10 is applied to the gate terminalsof the Qs21 to Qs24.

[0086] As the MOSFETs Qs31 to Qs33, MOSFETs of the N-channel type areused to supply the ground potential Vss to one of the terminals of eachof the capacitors C12 and C13 for boost and the boost capacitor Cb, anda clock PCLK2 having an amplitude of V0 as shown in FIG. 10 is appliedto the gate terminals of the MOSFETs. The phase of the clock PCLK1 isopposite to that of the clock CLK2 having the amplitude V0 to be appliedto the reference side terminal of the capacitor C11 for boost at thefirst stage, and the clock PCLK2 has the same phase as that of PCLK1.Although not limited, each of the clocks CLK2, PCLK1, and PCLK2 has acycle of 100 nS which is double of the cycle of the operation clock CLK1of the capacitor parallel type charge pump CP11.

[0087] The charge pump of the embodiment uses, as the operation voltageV0, the boosted voltage VWDP generated by the capacitor parallel typecharge pump CP11. In a period T11 in which the clock CLK2 to be appliedto the reference side terminal of the capacitor C11 for boost at thefirst stage is at the low level, by changing the clocks PCLK1 and PCLK2to the high level and turning on the switch MOSFETs Qs21 to Qs24, Qs31and Qs32, the capacitors C11, C12, and C13 for boost are precharged toV0. After that, the switch MOSFETs Qs21 to Qs24, Qs31, and Qs32 areturned off, both terminals of each of the capacitors C11, C12, and C13for boost are interrupted from the source voltage terminal and theground terminal, and the clock CLK1 is changed to the high level. As thepotential of the node on the ante stage side, that is, the sourcepotential becomes higher than the gate potential (=V0), the switchMOSFETs Qs11 and Qsl2 are automatically turned on, and the capacitorsC11, C12, and C13 for boost are connected in series. As a result, aboosted voltage corresponding to an almost the total (=3V0) of theprecharge voltages of the capacitors C11 to C13 for boost is generated.

[0088] At this time, in the booster BST, simultaneously with theprecharging of the capacitors C11 and C13 for boost, the capacitor Cbfor boost is precharged to V0. When the clock CLK1 is changed to thehigh level, like the switches MOSFETs Qs11 and Qs12, the MOSFET Qs13 isturned on, thereby boosting the gate voltage of the switch MOSFET Qs13at the final stage to a potential higher than the potential (sourcepotential) of the node at the ante stage side by an amount of theprecharge voltage V0, so that the MOSFET Qs13 enters a sufficient ONstate. Consequently, the boosted voltage generated by the capacitors C11to C13 can be transmitted to a circuit at the post stage without apotential drop.

[0089] Since the basic configuration of the charge pump CP22 forgenerating the negative boosted voltage VEW out of the capacitor serialtype charge pumps CP21 and CP22 in the second booster 2 is similar tothat of FIG. 9 except that the polarity of a voltage generated isopposite to that of the CP 21, it is not shown.

[0090] The charge pump CP22 for negative voltage is different from thecharge pump for positive voltage in FIG. 9 with respect to the followingpoints. The conduction types of the MOSFETs used are opposite to eachother, that is, N-MOS is used instead of P-MOS in FIG. 9, and P-MOS isused in place of N-MOS. The amplitude of the clock PCLK2 is differentfrom that in FIG. 9. The direction of precharging of the capacitors C11to C13 for boosting which are connected in series is opposite to that inFIG. 9. The timings of the clocks CLK2, PCLK1, and PCLK2 are differentfrom each other. At the time of precharging, in the charge pump of FIG.9, the ground potential Vss is applied in advance to the reference sideterminal of the capacitor C11 for boost at the first stage and thevoltage is increased to Vcc in the boosting operation. In contrast, inthe charge pump for negative voltage, the source voltage V0 is appliedin advance to the reference side terminal of the capacitor C11 for boostat the first stage, and the voltage is dropped to the ground potentialVss in the boosting operation, thereby generating the negative voltage.

[0091] The method of applying the write voltage to the word line in thewriting mode and the configuration of the internal power generatingcircuit have been described above. In the flash memory of theembodiment, regarding the voltage to be applied to the bit line BL aswell, the method of applying the boosted voltage is devised.

[0092] In the flash memory of the embodiment, in the writing mode, ahigh voltage is applied to the word line WL (control gate) and, by usingthe FN tunnel phenomenon, negative charges are injected to the floatinggate of a memory cell to increase the threshold voltage of the memorycell. Consequently, according to write data, the bit line BL to which amemory cell desired to have the high threshold voltage (for example,data “1”) is connected is not precharged, that is, is set to 0V. On theother hand, the bit line to which a memory cell whose threshold voltageis not desired to be increased (for example, data “0”) is connected isprecharged to 5.5V. In the writing mode, the source of each selectionmemory cell is floated (open). In the specification, the voltage to beapplied to the bit line to which the memory cell whose threshold voltageis not desired to be increased is connected will be called a write checkvoltage.

[0093] As shown in FIG. 11, in the flash memory of the embodiment, inthe writing mode, the voltage of the word line WL is boosted in twolevels by being increased once to 7V and then to 18V by the controlcircuit 32. The voltage of the bit line BL is boosted to 5.5V in aperiod since the word line WL is raised to 7V until it is raised to 18V.The CPU 321 of the control circuit 32 performs such a control byoutputting the start signals ST1, ST2, and ST3 at predetermined timingsto the charge pumps CP11, CP12, and CP21 on the basis of detectionsignals LMD1 and LMD2 from the limiters LM1, LMT2, and LM3 of the chargepumps and an output of the timer.

[0094]FIG. 12 schematically shows a boosted voltage supply circuit ofFIG. 7. In FIG. 12, circuit blocks designated by the same referencenumerals as those in FIG. 7 denote the same circuit blocks. It isassumed that the charge pumps CP11, CP12, and CP21 shown in FIG. 12include the control gates 7 a, 7 b, and 7 c in FIG. 7 (374 in FIG. 4)and the control gate 375 in FIG. 4. SELL denotes a selector forselectively supplying the boosted voltage VWD generated by the chargepump CP11 to either the sense and latch circuit 11 or the charge pumpCP21 at the second stage, and SEL2 denotes a selector for selectivelysupplying the boosted voltage generated by the charge pump CP21 or CP12to the word driver 14 a. Each of the selectors SEL1 and SEL2 can beconstructed by an analog switch, and switch control signals CS1 and CS2are supplied from the CPU 321 in the control circuit 32.

[0095] When a write command is supplied, first, the CPU 321 changes thestart signal ST2 to the charge pump CP12 to the high level to start theboosting operation (timing t1). After elapse of a predetermined time,the CPU 321 changes the start signal S12 to the charge pump CP11 to thehigh level to start the boosting operation (timing t2). Since all of thebit lines BL (each having a parasite capacity of a few pF) are subjectsto be precharged, the load capacity is obtained by multiplying the loadcapacity by the number of bit lines (1 to 8×103). The number of wordlines WL to be selected is one, and the load capacity is a few pF.Consequently, even when the source voltage Vcc is at a low level such as1.8V, a desired boosted level (7V) can be achieved in a relatively shorttime. When it is regarded that the voltage is boosted on the basis ofnot the detection signal from the limiter LM3 but time, there is noharm.

[0096] In response to a change of the detection signal LMD1 from thelimiter LM1 of the charge pump CP11 to the high level (timing t3), theCPU 321 switches the switch signal CS1 to the selector SELL to supplythe boosted voltage VWD generated by the charge pump CP11 to the chargepump CP21, and changes the start signal ST3 to the charge pump CP21 tothe high level to start the boosting operation (timing t4). At thistime, the CPU 321 negates the start signal ST2 to the charge pump CP12.After that, when the boosted voltage VWW of the charge pump CP21 reachesa predetermined level (18V), the detection signal LMD2 from the limiterLM2 goes high (timing t5). The switch signal CS2 to the selector SEL2 istherefore switched to supply the boosted voltage VWW generated by thecharge pump CP21 to the word driver 14 a.

[0097] In the flash memory of the embodiment, the timer 323 is activatedon start of the boosting operation of the charge pump CP21 in responseto the start signal ST3 from the CPU 321. When the timer counts up thepredetermined time before the detection signal LMD2 from the limitercircuit LM2 goes high, the CPU 321 switches the switch signal CS2 to theselector SEL2 to supply the boosted voltage VWW generated by the chargepump CP21 to the word driver 14 a. When the boosted voltage VWW issupplied to the word driver 14 a, the timer 323 starts counting a timeof application of the write voltage. When the CPU 321 determines thatthe predetermined time has elapsed on the basis of the timer, the CPU321 negates the start signals ST1 and ST3 to the charge pumps CP11 andCP21.

[0098] As described above, according to the embodiment, in the writingmode, the word line WL is raised in two levels. When the precharge ofthe bit line BL is finished, the charge pump CP21 at the second stage isstarted. The charge pump CP11 for generating the boosted voltage forprecharging the bit line BL also serves as the charge pump at the firststage of the booster for word line. Consequently, at the second andsubsequent writing operations performed before the threshold voltagereaches a target value in write verifying, the rise time (from t2 to t3in FIG. 11) of the charge pump CP11 becomes shorter, so that the timerequired to boost the word line becomes shorter and the word line can beprevented from being started to be boosted before the bit line BLreaches a correct precharge level (5.5V).

[0099] As a result, a change in the threshold voltage of a not-selectedmemory cell, called a “disturb”, can be prevented. Specifically, like inthe embodiment, in the case of boosting the word line after elapse ofthe predetermined time even if the boosted voltage of the charge pumpfor raising the word line does not reach the target level, if no countermeasure is taken, before the bit line BL reaches the correct prechargelevel (5.5V) but is, for example, at 4.5V, the word line is started tobe boosted, and a disturb may occur. In the embodiment, however, afterconfirming that the boosted voltage for the bit line has reached thetarget level, the boosting operation of the charging pump at the secondstage for boosting the word line is started by the boosted voltage ofthe charge pump which generates the voltage. Thus, it can be avoidedthat the boost of the word line is started before the bit line BLreaches the correct precharge level (5.5V).

[0100] As described above, in the flash memory to which the invention isapplied, even if the source voltage Vcc is at a low level such as 1.8Vand the boosted voltage of the charge pump does not reach the targetlevel, the writing operation is started on the basis of time.Consequently, it can be prevented that the program cannot be escapedfrom the writing mode. Moreover, even when the potential of the wordline is lower than the target level more or less, by increasing thetime, that is, the number of writing times, data can be written to thememory device of the flash memory. Although there is a drawback that thewriting time becomes longer a little, the memory device of the flashmemory has an advantage that the correct writing can be performed.

[0101] Among portable electronic devices often operated on batteries, astorage medium for a recording/reproducing device for music is oftenused in such a manner that reproduction is performed by a reproducerdriven on a battery and recording is executed by a device of whichsource voltage of 3.3V or higher is guaranteed. For example, in aportable electronic device driven on a battery of a low level such as1.8V, even when data cannot be recorded, that is, written to the storagemedium, there is little harm. On the other hand, it is expected that avoice recorder used for recording conversation in normal time or thelike usually records voice by a device driven on a battery at a lowlevel of 1.8V or the like. It is therefore inconvenient for the storagemedium used for the voice recorder that if writing operation cannot beperformed with the low source voltage. In the voice recorder, ascompared with the recording/reproducing device for music, the dataamount is small, so that the high speed is not so required. It can betherefore said that the flash memory to which the invention is appliedis optimum as a storage medium used for the voice recorder. Moreover,according to the embodiment, a product (flash memory) which can be usedfor both a system operated by the source voltage Vcc of 3V and a systemoperated by the source voltage Vcc of 1.8V can be provided. A medium onwhich voice is recorded by the voice recorder can be also reproduced bya stay-at-home audio device.

[0102]FIG. 13 shows an example of the internal configuration of astorage medium such as a memory card or a SmartMedia in the case wherethe flash memory of the embodiment is used as a storage medium for thevoice recorder. The storage medium may be of an inner type that aconnection terminal is not exposed to the outside. The storage mediaaccording to the invention include not only a storage medium of what iscalled a card type but also media of all types which can be loaded in anelectronic device, such as a stick-shaped medium.

[0103] A storage medium 100 of the embodiment is constructed by,although not limited, housing a single flash memory chip FLM and acontroller chip CONT having the functions of generating an addresssignal and a control signal to an interface with the outside and theflash memory, generating an ECC, and making a check in a package made ofplastic or the like or mounting the flash memory chip FLM and thecontroller chip CON on a printed wiring board, and molding the whole byusing a resin or the like.

[0104] The controller chip CONT is constructed by a dedicated LSI, agate array, or the like and is connected to the flash memory chip FLMvia a control signal line group (bus) 111 for sending control signalsand an address and data signal line group (bus) 112 for transmitting theaddress signal, data signal, and a command code in a time divisionmanner and also connected to a terminal 120 for external connection asan electrode inserted to come into contact with a card slot of anexternal portable electronic device or the like. An access to the flashmemory chip FLM is made via the controller chip CONT. In place of thecontroller chip CONT, a general microcomputer may be used to generate acontrol signal or the like to be supplied to the memory chip FLM.

[0105] In FIG. 13, reference numeral 121 denotes an external powersource terminal to which the source voltage Vcc to be supplied to thecontroller chip CONT and the flash memory chip FLM is applied, and 122indicates an external ground terminal to which the ground potential isapplied. Although the storage medium of the embodiment obtained byhousing one controller chip CONT and one flash memory chip in the singlepackage has been described as an example, a storage medium can be alsoconstructed by only a single flash memory, or by a single controllerchip CONT and a plurality of flash memory chips. In addition to thecontroller chip CONT, a CPU may be also housed. A storage medium may bealso constructed by housing only a single chip microcomputer havingtherein a flash memory in a package.

[0106]FIGS. 14A to 14C show a configuration according to a use state ofa voice recorder as an example of an applied system using the card typestorage medium. FIG. 14A shows a schematic configuration in the case ofwriting voice data to a storage medium. FIG. 14B shows a schematicconfiguration in the case of reproducing the voice data from the storagemedium. FIG. 14C shows a schematic configuration in the case of editingor processing the voice data recorded in the storage medium. Shown inthe FIGS. 14A to 14C are a voice recorder body 310, a microphone 320, abattery 330, a headphone or earphone 340, and a personal computer 350.In such a system, the storage medium 100 having therein the flash memoryas shown in FIG. 13 is inserted to a medium insertion portion providedin the voice recorder body 310 and is used.

[0107]FIG. 15 shows a schematic configuration of an internal circuit ofthe voice recorder.

[0108] As shown in FIG. 15, the voice recorder body 310 includes amedium insertion portion 311 for removably holding the storage medium100, an input and record circuit 312 for converting a sound electricsignal input from the microphone 320 into a digital signal and writingthe digital signal to the storage medium 100, a reproduction/outputcircuit 313 for reading the sound digital data from the storage medium100, converting the sound digital data into an analog signal, andgenerating and outputting a signal for driving the headphone or earphone340, input operation means 314 such as a touch panel used by theoperator or user to give an instruction from the outside, and acontroller 315 for controlling the recording circuit 312, thereproducing circuit 313, and the like in accordance with the instructionfrom the input operation means 314.

[0109] Another example of an applied system using a card type storagemedium is an on-line music distributing system. The on-line musicdistributing system is a system for transmitting music data from a hostcomputer to a terminal device via a telephone line and a communicationnetwork such as the Internet, storing the music data into a storagemedium on the terminal device, inserting the storage medium into aportable device, reading the data, and reproducing the music. As thestorage medium in the system, a medium having therein a flash memory asshown in FIG. 13 is used.

[0110] A music distributing system of, other than the method usingcommunication, a method writing music data stored in a storage of acomputer in a CD shop or the like into the storage medium is alsoproposed. In such a case, high-speed writing is requested. It istherefore improper to write data by using a power source of the batteryof 1.8V or the like to the storage medium having therein the flashmemory according to the embodiment. In this case, however, since thedata is written in a CD shop, it is sufficient to use a source voltageobtained by converting a commercial power source of AC 100V into a DCvoltage such as 3.3V by an AC adapter or the like.

[0111] On the other hand, in the case of the music distributing systemusing communications, the communication speed is not so high at present.When a portable electronic device for recording/reproducing data to/froma storage medium is connected to a personal computer at home to writereceived music data into the storage medium, the recording can beperformed by driving the portable electronic device on a battery sourcewithout using a source voltage of an AC adapter.

[0112] The invention achieved by the inventors has been specificallydescribed above on the basis of the embodiment. Obviously, the inventionis not limited to the embodiment but may be variously modified withoutdeparting from the gist. For example, the embodiment has been describedthat time data stored in the ROM is set in the timer and the timerstarts the counting operation. The counting operation maybe performed byusing time data set in a register in place of the ROM.

[0113] Although the case where the invention achieved by the inventorsis applied to the flash memory in the utilization field as thebackground has been described above, the invention is not limited to thecase but can be widely used for a semiconductor memory requiring aplurality of boosted voltages and internally generating the boostedvoltages. The invention is effective for application to a nonvolatilesemiconductor memory for writing/erasing data to/from a nonvolatilememory element having a floating gate by using the FN tunnel phenomenonand is particularly effective for application to a multi-value memoryfor storing information of a plurality of bits in one element.

[0114] Effects obtained by a representative one of the inventionsdisclosed in the specification will be briefly described as follows.

[0115] According to the invention, the semiconductor memory capable ofavoiding the situation that the program cannot be escaped from thewriting operation and promptly finishing the writing operation inaccordance with the level of an external source voltage is obtained. Asa result, the throughput of the system using the semiconductor memorycan be improved as a whole.

What is claimed is:
 1. A semiconductor memory having a plurality ofmemory cells and a booster for generating a boosted voltage on the basisof a source voltage supplied from the outside, for applying said boostedvoltage to a selected memory cell when the voltage generated by thebooster reaches a predetermined level, comprising: a voltage detectingcircuit for detecting whether said boosted voltage has reached apredetermined potential or not; a control circuit capable of controllingstart and stop of a boosting operation of said booster; and a timercapable of counting predetermined time, wherein said control circuitapplies said boosted voltage to a selected memory cell when said voltagedetecting circuit detects that said boosted voltage has reached thepredetermined potential and, when it is detected on the basis ofcounting information of said timer that the predetermined time haselapsed since said booster started the boosting operation, said controlcircuit applies the boosted voltage to said selected memory cell even ifthe boosted voltage generated by said booster has not reached thepredetermined potential yet.
 2. The semiconductor memory according toclaim 1, further comprising switching means for supplying orinterrupting said boosted voltage, wherein said switching means receivesa control signal from said control circuit and applies said boostedvoltage to said selected memory cell.
 3. The semiconductor memoryaccording to claim 2, further comprising a clock generating circuit forgenerating an internal clock signal.
 4. The semiconductor memoryaccording to claim 3, wherein said booster operates on the basis of aclock signal for boosting generated by said clock generating circuit,and said timer operates on the basis of a clock signal for countinggenerated by said clock generating circuit.
 5. The semiconductor memoryaccording to claim 4, further comprising a plurality of word lines,wherein each of said plurality of memory cells is connected to acorresponding word line, and said boosted voltage is applied to a wordline to which the selected memory cell is coupled.
 6. The semiconductormemory according to claim 5, wherein said booster has a positive voltagegenerating circuit for generating a positive high voltage on the basisof a source voltage from the outside and a negative voltage generatingcircuit for generating a negative high voltage on the basis of thesource voltage from the outside.
 7. The semiconductor memory accordingto claim 6, wherein each of said plurality of memory cells has athreshold voltage corresponding to data to be stored.
 8. Thesemiconductor memory according to claim 7, wherein data iswritten/erased to/from each of said plurality of memory cells by using atunnel phenomenon.
 9. A semiconductor memory having a plurality ofmemory cells each having a threshold voltage corresponding to data to bestored, a plurality of bit lines, a plurality of data latches providedfor the bit lines, and a booster for generating a boosted voltage on thebasis of a source voltage supplied from the outside, each of saidplurality of memory cells being coupled to a corresponding bit line,said boosted voltage being applied to said memory cell selected whensaid boosted voltage reaches a predetermined potential, and data beingrewritten to the memory cell in accordance with data latched by saiddata latch, comprising: a voltage detecting circuit for detectingwhether said boosted voltage has reached a predetermined potential ornot; a control circuit capable of controlling start and stop of aboosting operation of said booster; and a timer capable of countingpredetermined time, wherein said control circuit applies said boostedvoltage to a selected memory cell when said voltage detecting circuitdetects that said boosted voltage has reached the predeterminedpotential and, when it is detected on the basis of counting informationof said timer that the predetermined time has elapsed since said boosterstarted the boosting operation, said control circuit applies the boostedvoltage to said selected memory cell even if the boosted voltagegenerated by said booster has not reached the predetermined potentialyet.
 10. The semiconductor memory according to claim 9, wherein each ofsaid memory cells stores multi-value information by having any one of aplurality of threshold voltages.
 11. The semiconductor memory accordingto claim 10, further comprising switching means for supplying orinterrupting said boosted voltage, wherein said switching means receivesa control signal from said control circuit and applies said boostedvoltage to said selected memory cell.
 12. The semiconductor memoryaccording to claim 11, further comprising a clock generating circuit forgenerating an internal clock signal, wherein said booster operates onthe basis of a clock signal for boost generated by said clock generatingcircuit, and said timer operates on the basis of a clock signal forcounting generated by said clock generating circuit.
 13. Thesemiconductor memory according to claim 12, further comprising aplurality of word lines, wherein each of said plurality of memory cellsis connected to a corresponding word line, and said boosted voltage isapplied to a word line to which the selected memory cell is coupled. 14.The semiconductor memory according to claim 13, wherein a second voltageboosted by said booster in accordance with data latched by said datalatch and reached a predetermined potential is applied to said bit line,and said control circuit reserves activation of said timer until saidsecond voltage reaches the predetermined potential.
 15. A systemsupplied with voltage from outside, comprising a controller and anonvolatile memory device which includes: a nonvolatile memory array; avoltage generating circuit generating a boosted voltage from saidsupplied voltage and supplying said boosted voltage to said nonvolatilememory array, and wherein said voltage generating circuit includes atimer circuit and voltage detecting circuit, wherein said timer circuitdetects a first event which is a predetermined time having passed from astart of voltage boosting, wherein said voltage detecting circuitdetects a second event which is a voltage level of said boosted voltagehaving reached a predetermined voltage level, and wherein said voltagegenerating circuit starts supplying said boosted voltage when said timercircuit detects said first event or said voltage detecting circuitdetects said second event.
 16. A system according to claim 15, whereinsaid nonvolatile memory device is removable from said system.
 17. Asystem according to claim 15, wherein said boosted voltage is used forprogramming data to said nonvolatile memory array.
 18. A systemcomprising: a voltage supply source; a controller; and a nonvolatilememory device, wherein said voltage supply source supplies voltage tosaid controller and said nonvolatile memory device, wherein saidcontroller is capable of supplying a program command to said nonvolatilememory device, wherein said nonvolatile memory device includes aplurality of nonvolatile memory cells and a voltage generating circuitand carries out a data storing operation for storing data to ones ofsaid nonvolatile memory cells responsive to said program command,wherein said voltage generating circuit comprises a timer circuit, avoltage detecting circuit and a voltage output circuit and generates aboost voltage boosted from said supplied voltage, wherein said timercircuit outputs a first signal to said voltage output circuit when apredetermined time has passed from a start of voltage boosting, whereinsaid voltage detecting circuit outputs a second signal to said voltageoutput circuit when a voltage level of said boost voltage has reached apredetermined voltage level, and wherein said voltage output circuitoutputs said boost voltage to said nonvolatile memory cells whenreceiving said first signal or said second signal.